The traditional method of coating sheet metal is to apply a solvent or water-based coating to the sheet metal, and then move the sheet metal through an oven to cure the coating. The curing is accomplished by heating the sheet metal and the coating to a temperature at which the solvent or water in the coating is evaporated and at which the coating itself is not harmed. To obtain a uniform coating and for greatest efficiency and lowest cost, the curing is a continuous process in which 1) the oven needs to be maintained at a constant temperature, 2) the sheet metal needs to be continuous and 3) the sheet metal needs to be moved through the oven at a constant rate of speed. A typical rate is 200 to 400 feet per minute.
To meet the requirement that the sheet metal be continuous, two pay off coils of sheet metal and a welder/joiner need to be provided. The sheet metal is advanced from one of the pay off coils, and when the end of that coil is reached, it is welded to the beginning of the second pay off coil. Then, as sheet metal is advanced from the second coil, the exhausted first coil is replaced. In addition, after the coating on the sheet metal is cured, the sheet metal needs to be re-coiled. To accomplish this, a shear and two re-coil mechanisms need to be provided. The continuously moving sheet metal is wound on a first re-coil mechanism, and when the desired coil size is reached, the sheet metal is cut by the shear. The sheet metal is then wound on the second re-coil mechanism while the coil on the first re-coil mechanism is removed.
Both the welding/joining operation at the beginning of the process and the shearing operation at the end of the process require interruption of the movement of the sheet metal. Therefore, to satisfy the requirement that the sheet metal move through the oven at a constant rate of speed, it is necessary that a first excess length of sheet metal be provided after the welder/joiner. This excess length of sheet metal is fed into the process during the time that the welding/joining operation is taking place and no sheet metal is being advanced from one of the pay off coils. It is apparent that even at the modest rate of speed of 200 to 400 feet per minute, allowing for the worst case interruption requires that the excess length of sheet metal be substantial.
This excess length of sheet metal is provided by apparatus referred to as an accumulator. This accumulator is typically a tower within which the excess sheet metal is vertically looped back and forth on itself in serpentine fashion. The ends of the loops are wrapped around rollers that move toward one another to shorten the loops when excess length is being used to replace the sheet metal not being provided by a pay off coil. Once the welding/joining operation is completed, the rollers move away from one another as the desired excess length is restored in the accumulator.
A second accumulator needs to be provided before the shear and re-coil mechanisms. This is because the sheet metal is continuously moving out of the oven at the constant rate of speed. During the time when the winding of the sheet metal is being transferred from one of the re-coil mechanisms to the other, the sheet metal needs to be accumulated. In the second accumulator, the length of the loops are increased when no re-coil mechanism is in operation and decreased when a re-coil mechanism is in operation. The oven used in the traditional method needs to be of considerable length in order to effect complete curing of the coating applied to the sheet metal. A length of 100 feet for sheet metal of 0.050 thickness moving at a rate of 200 to 400 feet per minute is appropriate. Of course, the length of the oven needs to be increased if the thickness of the sheet metal and/or the rate at which the sheet metal moves through the oven is increased.
Prior to the coating being applied to the sheet metal, the sheet metal needs to be cleaned to remove contaminants that may interfere with the coating adhering to the sheet metal and pretreated to promote adhesion of the coating to the sheet metal. In the traditional method, both the materials used in the cleaning of the sheet metal and the materials used in the pretreating of the sheet metal contain pollutants in the form of volatile organic compounds that are emitted during the cleaning and pretreating operations. These pollutants need to be removed before the emissions can be released to the atmosphere.
Similarly, the coating used in the traditional method contain volatile organic compounds that are emitted during the curing operation. Again, these pollutants need to be removed before the emissions can be released to the atmosphere. It is, therefore, necessary in the traditional method to have in place pollution control equipment that removes these pollutants from the emissions exhausted from the cleaning operation, the pretreating operation, and the curing operation.
It is seen from the above that the traditional method for coating sheet metal has many deficiencies. First, it requires large amounts of equipment and a building of substantial size to house the equipment. Thus, it requires a significant investment of capital. Second, without a substantial increase in capital investment, it is a relatively low speed process. Third, it only lends itself to long runs. A coating line needs to operate around the clock for days or weeks once coating of sheet metal with a coating of a particular color has begun. Fourth, because of this and the many pieces of equipment involved in this process, the cost of operation and maintenance is significant. Last, and most importantly, it requires the installation of pollution control equipment to prevent pollution of the atmosphere.